Until recently, nanostructured devices have been limited to processors, memory chips and other integrated circuits devices, which have been produced using microlithography extended in nanodimensions—nanolithography. During the last few years a variety of new fields and applications have been brought to life due to maturation of nanolithography technique and wider access to nanolithography tools and foundries. One of the most promising new fields is nanophotonics.
Nanophotonic devices exploit the unique interaction of light with deep-subwavelength nanoscale objects. This relatively new class of highly compact, high-performance optical components is readily customized and easily integrated with other optical devices and electronics and is of significant interest to both electro-optic circuit designers and manufacturers. For visible light (used for digital imaging and display applications) and near-infrared (IR) wavelengths (used for some optical storage, sensor, and communications applications), this requires a capability of creating structures with dimensions on the order of tens to a few hundreds of nanometers with accuracy of 10 nm or less.
Fabricating physical devices with such fine-scale structures in a way that can be flexibly applied to a large variety of structural shapes and substrates presents the challenge of developing nanolithography techniques that support high-fidelity pattern replication with accuracies of a single nanometer. In general, materials can be formed into nanoscale structures by either bottom-up methods (built or grown molecule by molecule) or top-down methods (by etching the pattern into a deposited material).
One very promising application is a nanostructured anti-reflective coating (referred to as “AR coating”). Traditional thin-film AR coatings can suffer catastrophic failure or delamination from high-energy or thermal-cycling applications. High-power laser applications require low-reflectivity lenses to limit high-energy retroreflection. The thermal performance of these AR-coated substrates is governed by the composite structure's ability to dissipate heat generated by the absorption of incident laser energy during transmission or reflection. This ability is directly related to the absorption that takes place in the substrate, coating material, and various interfaces. Surface contamination, poor adhesion, and a mismatch in thermal properties can further contribute to the creation of nonuniform temperature distributions that gradually lead to film degradation, including cracking, peeling, delamination, and surface breakdown.
One approach that has shown great promise for achieving the increasingly high-performance requirements of AR surfaces is the use of motheye, or subwavelength, structures. The surface of a moth's eye is covered by an array of conical protuberances 200 nm high separated by 200 nm. A motheye structure creates what is effectively a gradient-index film from a material of uniform refractive index. Bruce MacLeod at Holographic Lithography Systems, Inc. (Bedford, Mass.) used holographic lithography for fabricating motheye-type structures. A further discussion of holographic lithography to fabricate motheye-type structure is found in Mr. MacLeod's article “Thin Films—Motheye Surfaces Reflect Little Eye”, published in Laser Focus World, August 1999, which is hereby incorporated by reference in its entirety. Holographic lithography is the process of recording, in a photosensitive film, a periodic pattern resulting from the interference of two coherent laser beams. The main difficulty with holographic exposure technique is to achieve structures with a high aspect ration homogeneously over the whole area.
G. Xie suggests another method of fabrication nanostructured anti-reflective layers: replication from natural biotemplate; his article “The fabrication of subwavelength anti-reflective nanotsructures using a bio-template” was published in Nanotechnology journal, v. 19 (2008). Specifically, the nano-nipple arrays on the surface of cicada wings have been precisely replicated to a PMMA (polymethyl methacrylate) film with high reproducibility by a technique of replica molding, which mainly involves two processes: one is that a negative Au mold is prepared directly from the bio-template of the cicada wing by thermal deposition; the other is that the Au mold is used to obtain the replica of the nanostructures on the original cicada wing by casting polymer. The reflectance spectra measurement shows that the replicated PMMA film can considerably reduce reflectivity at its surface over a large wavelength range from 250 to 800 nm, indicating that the anti-reflective property has also been inherited by the PMMA film.
A. Piehl, in U.S. Pat. No. 7,170,666 B2, tries to overcome optical lithography difficulties by fabrication nanostructured anti-reflective surfaces using self-assembly operation. He deposits thin gold layer and then upon heating to high temperatures converts this layer into plurality of nanostructures smaller than a wavelength of light.
Abovementioned methods of nanostructured anti-reflective layers fabrication are not manufacturable on industrial scale, scalable or production-worthy. Holographic lithography is too sensitive of a technique to be used for volume production. The biotemplate method is limited by specific specimen pattern. Both methods are limited by the processing area of the specimen.
Recently, Nanoimprint lithography method has been suggested, for example by Z. Yu for subwavelength (nanostructured) anti-reflective coatings fabrication in his article “Fabrication of large area subwavelength antireflection structures on Si using trilayer resist Nanoimprint lithography and lift-off”, published in Journal of Vacuum Science and Technology, v. B21(6), 2003. Nanoimprint method is based on deformation of photoresist upon mechanical impact by the nanostructured mold. Two-dimensional (“2D”) subwavelength broadband anti-reflection surfaces on silicon have been demonstrated using this technology with reflectivity of 0.3% at 632.8 nm wavelength.
Another example of nanostructured devices is based on plasmonic structures. The optical properties of metal nanoparticles, especially those of the noble metals Au, Ag, and Cu, show striking differences in their optical response relative to their bulk or thin-film counterparts. The ability of such structures to sustain coherent electron oscillations known as surface plasmons (SPs) leading to electromagnetic fields confined close to the metallic surface has been intensively investigated both in light of the fundamental physics involved and for applications such as surface-enhanced spectroscopy and enhancement of a wide range of nonlinear optical phenomena, sensing, light detection and generation.
Plasmonic effects have been explored and shown very promising results for enhancement of efficiency of light absorption in solar cells and light extraction of light emitting diodes. Most recently, studies have shown that spherical Au nanoparticles with diameters of 50-100 nm deposited on crystalline Si p-n junction photodiodes increase the absorption of light over a broad spectral range via the interaction of the incident electromagnetic radiation with SP modes in the nanoparticles that gives rise to electromagetic field enhancements in the active region of the photodiode, as was demonstrated by D. Schaadt in his article “Enhanced semiconductor optical absorption via surface plasmon excitation in metal nanoparticles,” published in Appl. Phys. Lett. 86, 063106, 2005 Because the electromagnetic fields present in a semiconductor give rise to an optical transition rate proportional to the square of the electric field amplitude, the resulting increase in amplitude of the electromagnetic fields results in an increased photogeneration of electron-hole pairs, and consequently increased photocurrent current from the device. Although metallic nanostructures are preferred for this application because of their strong interact with light, our technique is also capable of generating semiconductor and insulating nanostructures. In addition to nanoparticles, nanowires and stripes can be generated by our proposed technique as well.
David A. Boyd, Mark L. Brongersma, and Leslie Greengard in US patent application 20050202185 have used the field enhancement to initiate and control photochemical reactions, including excited electron-hole pairs in a wide variety of materials systems. D. Derkacs in his paper “Improved performance of amorphous silicon solar cells via scattering from surface plasmon polaritons in nearby metallic nanoparticles”, published in Applied Physics Letters journal, v, 89, 09310, have applied this concept to a-Si:H solar cells to achieve engineered enhancements in optical absorption, short-circuit current density, and energy conversion efficiency. At relatively modest nanoparticle densities, increases in short-circuit current density and energy conversion efficiency were obtained under halogen lamp illumination in excess of 8%, with finite-element electromagnetic simulations indicating that substantially larger increases should be possible at higher nanoparticle densities. The researchers also noted, that in order to increase the concentration of nanoparticles on the surface, the deposition procedure was repeated up to five times; additional iterations typically resulted in clustering of nanoparticles on the surface. Obviously, this nanoparticle deposition approach has limitations in the achievable density of particles, which in turn limits plasmonic efficiency enhancement.
S. Pillai in his paper “Surface plasmon enhanced silicon solar cells”, published in Journal of Applied Physics, v. 101, 093105 (2007) reported a sevenfold enhancement for wafer-based cells at λ=1200 nm and up to 16-fold enhancement at λ=1050 nm for 1.25 μm thin silicon-on-insulator (SOI) cells using plasmonic effect based on metal nanoparticles. He also reported a close to 12-fold enhancement in the electroluminescence from ultrathin SOI light-emitting diodes and investigated the effect of varying the particle size on that enhancement. Metal nanoparticles were deposited by thermal evaporation of thin layers of silver followed by annealing. During annealing process the particles coalesce together to form islands due to surface tension.
S. Fujimori in his paper “Plasmonic light concentration in organic solar cells” accepted in NANOLETTERS, P. 1-17, reported on nanoparticle plasmonic enhancements for organic solar cells. He used an electrostatically-assisted aerosol deposition technique to deposit gold nanoparticles, which showed improvements in power conversion efficiency of up to 40%. Even further enhancements were expected by these authors for an increased coverage of well-dispersed Au nanoparticles. Unfortunately, increased coverage was not possible with the presented approach due to the high probability of nanoparticle clustering. Moreover, control of the metal nanostructure shape and organization of the metal nanoparticles, which is not possible with the employed technique, may lead to additional gains in efficiency.
Scott P. Price in his paper “Addressable, Large-Area Nanoscale Organic Light-Emitting Diodes” published in Small Journal, 2007, 3, No. 3, 372-374, has used soft nanolithography to produce patterns with feature sizes less than 500 nm using composite poly(dimethylsiloxane) (PDMS) stamps. He reported the fabrication of nano-OLED arrays over relatively large areas (about cm2) and with higher pixel densities than those prepared using serial techniques. The method is very promising, but this implementation still limits processing area by actual size of the stamp, and does not allow high-throughput fabrication of large areas of optical materials.
Jing Zhao in his paper “Localized surface plasmon resonance biosensors”, published in Nanomedicine 2006, 1(2), P. 219-228, have demonstrated that metallic nanoparticle arrays can serve as optical sensor platforms with submonolayer sensitivity for (bio)chemical molecules. The preferred way of making these structures over large areas is by nanosphere lithography. This process requires multiple consecutive deposition, washing, and etching steps and is not suitable for mass production.
Nanostructured surfaces have also been proved very useful in fabricating so called self-cleaning coatings. Peter Forbes in his article “Self-cleaning materials: Lotus-inspired nanotechnology”, published in Scientific American, Jul. 30, 2008, explains principle of self-cleaning materials based on Lotus-Leaf effect: superhydrophobicity created by nature using nanopillar arrays and hydrophobic materials. Roach in his article “Progress in superhydrophobic surface development”, published in Soft Matter, 2008, 4, P. 224-240 describes, for example, numerous methods of achieving self-cleaning effect based of superhydrophobic surfaces. Scaling up of these technologies for industrial applications are very problematic. Michael Berger in his article “Moth eyes self-cleaning antireflection nanotechnology coatings, published in Nanowerk, 2008 reports on Moth eye type coatings, which can combine anti-reflective and self-cleaning properties. Such coatings have been fabricated using colloids of silica particles deposition followed by reactive ion etch.